Conventionally, an oil pressure control for an automatic transmission has been widely known, which controls engagement and disengagement of a friction engagement element (a friction clutch and a friction brake) by adjusting an oil pressure to be supplied to the friction engagement element from an oil pressure source via a solenoid valve. In Japanese Patent Laid-Open Publication No. 2002-295529, a method of learning a clutch torque point is described for controlling the engagement and disengagement of the friction engagement element. More particularly, an electronic control unit learns a duty ratio of a duty pulse as a torque transmission initiating point (a torque point) and appropriately controls the opening operation of the solenoid valve. Therefore, the clutch can be rapidly operated to be engaged within a region before the clutch engagement. The clutch can be then completely engaged at a relatively slow engagement speed after the clutch engagement.
As illustrated in FIG. 8, an oil pressure pre-charge is performed for rapidly charging the friction engagement element with the fluid while a clutch piston (hereinafter, referred to as a piston) has been within an initial range around a forward portion of the piston. Therefore, the piston can be rapidly operated. It is necessary to improve response and traceability of the clutch after a predetermined pre-charge period of time. Therefore, the piston operating speed is decelerated about zero immediately before the engagement of the friction engagement element. The oil pressure supplied to the piston is maintained at a low oil pressure (a standby oil pressure) corresponding to force of a return spring, wherein the piston is maintained at a position. The return spring is considered as a reaction force element of the piston. As described above, the shift operation can be operated much faster and the shift shock can be avoided.
However, the spring force of the return spring varies depending on the return springs. The above described oil pressure control is different from a general oil pressure. That is, the piston stroke is controlled by the amount of fluid to be supplied to the piston. Therefore, performance specifications such as a discharge amount of oil from an oil pump affect on the clutch engagement, wherein an actually supplied pressure value can not correspond to a target pressure value. Therefore, recent developments have led to suggest a method of detecting and setting the standby oil pressure with high precision. In this case, vehicle individual differences due to fluctuations of automatic transmission, engines, and solenoid valves can be absorbed. Therefore, a vehicle or a transmission with a stabilized quality can be supplied.
As a technology for setting the standby oil pressure considering the vehicle individual differences, Japanese Patent Laid-Open Publication No. 1996-338519 describes a fluid-type friction engagement element control apparatus which can monitor changes of an input shaft rotational speed and modify a pressure immediately before occurrence of the clutch engagement force, i.e., the standby oil pressure, based upon the input shaft rotational speed changes. Further, Japanese patent Laid-Open Publication No. 1996-303568 describes an automatic transmission control apparatus that can perform a proper neutral control and set an appropriate standby oil pressure based upon a clutch engagement initial pressure upon start of the clutch engagement.
However, according to the Publication No. 1996-338519, the input shaft rotational speed change is very small and may be detected in error due to fluctuations of engine rotational speeds and noise. Further, there may be a possibility that the input shaft rotational speed change may become unclear if the clutch disengagement control and the pre-charge control is performed inappropriately. Therefore, although the fluid-type friction engagement element control apparatus described above can be applied for learning for a long-period of time, the apparatus is not effective to be applied upon an initial delivery stage.
Further, according to the publication No. 1996-338519, as far as the performance of the automatic transmission varies depending upon automatic transmissions, it is necessary to change a threshold value applied for detecting the engagement initial pressure depending upon automatic transmissions. Still further, it is necessary to consider influences due to a change of temperature (oil temperature). Further, the apparatus disclosed above can be effectively applied for detecting the standby oil pressure of the friction engagement element such as a friction clutch C1 upon the shift operation from the N position to the D position, because the input shaft rotational speed change via the clutch C1 is equivalent or less than the input shaft rotational speed change of the engine. However, it is not appropriate for detecting the standby oil pressure at the other friction engagement elements, because the input shaft rotational speed change thereof is large.
A need thus exists for providing an improved automatic transmission and method of setting an automatic transmission standby oil pressure value to obtain a standby oil pressure value with high precision.